1. Field of the Invention
The present invention relates to a paint for forming an insulating film used in a plasma display panel, a plasma address liquid crystal display, or an image forming apparatus using electron-emitting elements, and to a plasma display panel in which an address electrode protective film and partition walls provided on its back plate and a dielectric layer provided on its front plate are formed using this paint. It is to be noted that the term xe2x80x9cpaintxe2x80x9d used in this application means a liquid composition using a volatile carrier and coating a components in which the coating components are left behind on a surface after the evaporation of the carrier.
2. Description of the Related Art
As an image forming apparatus that can be substituted for a heavy and large conventional cathode-ray tube, there has been increasing demand for a thin and light flat-type display or, as it is called, a flat panel display (FPD). A typical FPD is a liquid crystal display (LCD). However, LCDs have problems such that images obtained are dark, a viewing angle is narrow, and it is difficult to increase an area thereof. Therefore, in recent years, a plasma display panel (PDP) and an image forming apparatus using electron-emitting elements are attracting great attention and the demand for them has been growing because they can be increased in definition and in size further than LCD.
FIG. 1 shows one example of a structural view (cross-sectional view) of PDP. Hereinafter, the principle of display by PDP will be explained with reference to FIG. 1.
Between each pair of display electrodes 3 provided on a front glass substrate 1, plasmas are discharged to generate ultraviolet rays. The ultraviolet rays excite phosphors 6, which are separated from each other by partition walls 9 provided on a back glass substrate 2, to make the phosphors emit visible lights. The visible lights emitted from the respective phosphors 6 pass through an MgO film 10, a front dielectric layer 7, and the front glass substrate 1 to be displayed as an image. At this time, by applying signals to selected address electrodes 5 to specify the discharge cells to be displayed, a desired image can be obtained.
To generate and maintain plasmas, the electrodes need to be insulated from each other. The front dielectric layer 7 is provided to obtain an insulating effect. The front dielectric layer 7 is obtained as follows: a paint obtained by dispersing a resin and inorganic fine particles, which are glass powder mainly containing Si or the like, in an organic solvent is coated onto the front glass substrate, and the paint is dried and fired, thereby giving the dielectric layer 7. The organic solvent is vaporized during the drying process. During the firing process, the resin is vaporized and the glass powder melt to be joined to each other. The paint turns into a glass film when cooled after the firing, thus forming the front dielectric layer 7.
On the back glass substrate 2, the back dielectric layer 8 is provided for the purpose of protecting the address electrodes 5. The back dielectric layer 8 is formed in the same manner as that for the front dielectric layer 7.
Typically, an insulating film formed using glass is used as the front dielectric layer 7 and the back dielectric layer 8. For example, JP 2000-16835 A proposes using an insulation paste containing 70 to 95 wt. % of glass powder having an average particle diameter of 0.3 to 1.5 xcexcm and a maximum particle diameter of not more than 10 xcexcm and 5 to 30 wt. % of an organic component as a paste for forming thin insulating films as protective films provided on a front glass substrate and a back glass substrate and thick insulating films covering the electrodes on both the glass substrates.
As the phosphors 6, phosphors that emit lights of R (red), G (green), and B (blue), respectively, are used, and the partition walls 9 are provided to prevent these three colors from being mixed with each other. Like the front dielectric layer 7 and the back dielectric layer 8, the partition walls 9 are obtained also by coating a paint obtained by dispersing a resin and inorganic fine particles, which are glass powder mainly containing Si or the like, in an organic solvent, drying the paint, and then exposing/developing the paint in a desired pattern (photolithography) or etching the paint in a desired pattern by having fine particles of Zr or the like collide with the paint (sandblasting).
Known electron-emitting elements can be broadly classified into two types: hot cathode electron-emitting elements and cold cathode electron-emitting elements. In recent years, an image forming apparatus using cold cathode electron-emitting elements has been attracting attention because it does not consume powder during the standby period and can attain a high current density. The cold cathode electron-emitting elements can be classified into those of field emission type (FE type), metal/insulator/metal type (MIM type), and the like. In an image forming apparatus using cold cathode electron-emitting elements, phosphors emit lights upon being irradiated with electron beams emitted from the electron-emitting elements, thereby displaying an image.
Like the plasma display panel, the image forming apparatus using cold cathode electron-emitting elements also employs a glass insulating film. For example, a back glass substrate of the image forming apparatus includes a plurality of electron-emitting elements and wirings arranged in matrix for connecting these elements. These wirings are arranged in the X direction and Y direction to intersect with each other at the portions of the electrodes of the electron-emitting elements. To insulate the wirings and the electrodes at these intersections, a belt-shaped insulating film is needed. Such an interlayer insulating film is formed using, for example, glass containing lead oxides as a main component so that the thickness of the film becomes 10 to 100 xcexcm, preferably 20 to 50 xcexcm.
For example, JP 9(1997)-283060 A discloses that lower wirings are formed under an insulating film, a belt-shaped insulating film is then provided so as to intersect with the lower wirings at a right angle, and upper wirings are provided on the belt-shaped insulating film.
Therefore, like the plasma display panel, the image forming apparatus using cold cathode electron-emitting elements needs an insulating film, and the insulating film needs to have electrical insulating properties and exhibit a shielding effect as a protective film. Typically, such an insulating film is formed by a method such as vacuum evaporation, spattering, coating, or the like.
However, the methods utilizing a vacuum such as vacuum evaporation and spattering require costly production facilities. Especially in the case of manufacturing FPD, which seeks an increased area as described above, the manufacturing costs become even higher. Therefore, in FPD, reasonable coating methods such as screen printing and die coating commonly are used as a method of forming an insulating film.
However, when an insulating film is formed by the coating method, voids and/or pin-holes are generated in the resultant insulating film unless characteristics of the paint used for forming the film has been optimized. Thus, according to the coating method, it is difficult to obtain an insulating film producing a desired insulating effect. The voids and/or pin-holes are considered to be generated as a result of incorporation of foreign materials and/or air bubbles. Other than this, the insufficient wettability of the solvent contained in the paint is considered to be the major cause of the generation of the voids and/or pin-holes.
If the solvent has a poor wettability to the surface to be coated with the paint, crawling occurs, which accompanies the generation of cavities. Crawling is the phenomenon in which the paint is not spread evenly on the surface so that uncoated portions result. Therefore, the resultant insulating film is form ed unevenly and a desired insulating effect thus cannot be obtained. Further, when the surface to be coated with the paint has roughness, the chances of crawling occurring and the generation of cavities increase. Moreover, when the surface to be coated with the paint is formed of two or more materials, e.g., the case where Ag is formed on a glass substrate in a stripe-shape, the occurrence of crawling and the generation of the cavities become more remarkable. As a result, the resultant insulating film is formed still more unevenly so that some portions of the film have a thickness smaller than the other portions of the film. A desired insulating effect thus cannot be obtained.
Conventional paints for forming an insulating film generally contain xcex1-terpineol or butyl carbitol acetate as a solvent. However, these solvents do not have a good wettability to glass and thus causes crawling of the paint, which accompanies the generation of voids and/or pin-holes in the resultant film, separation of the film, etc.
Therefore, in conventional paints for forming an insulating film, firing characteristics of the paints have been controlled by adjusting the composition of the inorganic component, especially glass powder, to obtain a desired insulating effect. However, no attempt has been made to control the wettability of the paints by adjusting the composition of the solvent as a dispersion medium necessary to form the paints.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a paint for forming an insulating film, which has a good wettability to a surface to be coated with the paint and has no voids or pin-holes after being fired. It is also an object of the present invention to provide a plasma display panel in which dielectric layers provided on its back plate and front plate and partition walls provided on its back plate are formed using the paint and a method of manufacturing the same.
In order to solve the above-mentioned conventional problems, the inventors of the present invention have conducted a study on how to bring the action of a paint for forming an insulating film on a glass substrate, an ITO (indium tin oxide) film, Ag electrodes, etc., to the best condition to improve the ease of coating and the characteristics of the paint film. As a result, the inventors have found that, when the paint contains a solvent having a good wettability to inorganic materials such as a glass substrate, an ITO film, and Ag electrodes, crawling between the paint and the surface coated with the paint and generation of voids and/or pin-holes in the paint film hardly occur and separation of the paint film due to the insufficient adhesion between the paint and the surface thus can be prevented. From this fact, the inventors have found that plasma display panels having excellent voltage endurance characteristics can be obtained with high yield by forming a dielectric layer of a front panel and the like using this paint, thereby establishing the present invention.
Therefore, a paint for forming an insulating film on an inorganic material, which is at least one selected from glass, ceramic, and a metal compound, includes inorganic fine particles (a), a binder resin (b), and a solvent (c) that forms a contact angle of less than 50 with the inorganic material.
In the above-mentioned paint for forming an insulating film, it is preferable that the inorganic material contains glass, ceramic, and a metal compound. The paint for forming an insulating film according to the present invention exhibits a particularly excellent effect on such an inorganic material. More specifically, the glass is a glass substrate, the ceramic is an ITO film, and the metal compound is a metal electrode.
In the paint for forming an insulating film according to the invention, it is preferable that the solvent (c) contains 50 wt. % or more of at least one solvent selected from diethyl carbitol, dipropylene glycol-n-propyl ether, tripropylene glycol monomethyl ether, diethyl cellosolve, carbitol acetate, dibutyl cellosolve, and propylene glycol diacetate. By using the above solvents, the wettability of the paint to the inorganic material is improved.
Further, in the paint for forming an insulating film according to the invention, it is preferable that the solvent (C) contains 50 wt. % or more of at least one solvent selected from diethyl carbitol, dipropylene glycol-n-propyl ether, tripropylene glycol monomethyl ether, and diethyl cellosolve. By using the above solvents, the wettability of the paint to the glass substrate and the electrodes is made uniform even when the substrate has dirt thereon.
Furthermore, it is more preferable that the solvent (c) contains at least two solvents selected from diethyl carbitol, dipropylene glycol-n-propyl ether, tripropylene glycol monomethyl ether, and diethyl cellosolve. By using the above solvents, the generation of voids and/or cavities in the paint film is further suppressed and the voltage endurance characteristics are further improved.
In the paint for forming an insulating film according to the invention, the inorganic fine particles (a) preferably are at least one selected from SiO2, ZnO, B2O3, PbO, Bi2O3, BaO, P2O5, CaO, SrO, and MgO.
In the paint for forming an insulating film according to the invention, the binder resin (b) preferably is at least one selected from a cellulosic resin, an acrylic resin, polyvinyl alcohol, and polyvinyl butyral.
Further, it is preferable that the paint for forming an insulating film according to the invention contains 10 to 95 wt. % of the inorganic fine particles (a), 1 to 20 wt. % of the binder resin (b), and 4 to 85 wt. % of the solvent (c).
The paint for forming an insulating film according to the invention preferably is used for forming an insulating film used in a plasma display panel, a plasma address liquid crystal display, or an image forming apparatus using electron-emitting elements. The insulating film serves as an address electrode protective film or partition walls formed on a back plate, or a dielectric layer formed on a front plate of a plasma display panel.
Next, a method of manufacturing a plasma display panel according to the present invention includes the acts of coating a paint for forming an insulating film onto a member of a plasma display panel, the paint including inorganic fine particles (a), a binder resin (b), and a solvent (c) that forms a contact angle of less than 5xc2x0 with an inorganic material; and firing the paint to form an insulating film on the member.
In the above-mentioned method of manufacturing a plasma display panel, it is preferable that the member is a back plate, which is provided with address electrodes, of a plasma display panel and the insulating film serves as an address electrode protective film.
Further, in the above-mentioned method of manufacturing a plasma display panel, it is preferable that the member is a back plate of a plasma display panel and the insulating film serves as partition walls. Typically, the partition walls are formed on the address electrodes formed on the back plate of the plasma display panel.
Furthermore, in the above-mentioned method of manufacturing a plasma display panel, it is preferable that the member is a front plate, which is provided with display electrodes, of a plasma display panel and the insulating film serves as a dielectric layer.
Still further, it is preferable that the member such as a back plate and a front plate of a plasma display panel is cleaned by ultraviolet cleaning before being coated with the paint. Since organic dirt on the glass substrate and the electrodes can be removed by the ultraviolet cleaning, the wettability of the paint to the glass substrate and the electrodes can be improved, thereby suppressing the occurrence of crawling.
Next, a plasma display panel according to the present invention includes an insulating film that is obtained by coating a paint for forming an insulating film onto an inorganic material, the paint including inorganic fine particles (a), a binder resin (b), and a solvent (c) that forms a contact angle of less than 5xc2x0 with the inorganic material.
In the above-mentioned plasma display panel, it is preferable that the insulating film serves as an address electrode protective film.
In the above-mentioned plasma display panel, it is preferable that the insulating film serves as partition walls.
In the above-mentioned plasma display panel, it is preferable that the insulating film serves as a dielectric layer.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.